1. Field of the Disclosure
The disclosure relates to an electro-hydraulic pressing device, particularly for producing tube connections by means of press fittings, and for producing crimp connections in electrical engineering.
2. Discussion of the Background Art
Such a pressing device, as described e.g. in DE 20 2004 000 215, comprises e.g. two pliers-like press jaws. These are actuated by an electro-hydraulic drive. For this purpose, a pressure piston is provided in a piston chamber, the pressure chamber of the pressure piston being connected to the hydraulic pump. In this arrangement, conveyance of fluid into the pressure chamber will cause the pressure piston to move. This will result in the closing of the press jaws, wherein the pressure piston, optionally via a roller head, will actuate the press jaws. Further, alternatively to pliers-like press means, also so-called press loops are known. These comprise a plurality of mutually articulated press jaws which will be laid e.g. around a press fitting for pressing the same. The opening of the press loop will be connected to a pressing device so that, by contracting the press loop, a press connection will be realized. The pressing device used herein is designed substantially corresponding to the pressing device described in DE 20 2004 000 215. Further, axial pressing devices are known, in which the pressure build-up is also generated electro-hydraulically for displacement of a pressure piston.
For pressure build-up in the pressure chamber, hydraulic fluid is conveyed into the pressure chamber by the hydraulic pump. Thereby, the pressure piston is moved and the pressing is performed. If the pressing has been performed in a reliable manner, a switch-off pressure in the pressure chamber at the end of the pressing. Once the switch-off pressure has been reached, the pressing process will be automatically terminated. This is achieved by provision of an overpressure valve in the pressure piston. Said valve can be e.g. a needle valve as described in DE 20 2004 000 215 which, when the defined switch-off pressure has been reached, will open a connection channel provided in the pressure piston. When the switch-off pressure has been reached, the piston of the overpressure valve will be advanced into the pressure piston against the force of a bias spring, thus causing fluid to flow from the pressure chamber through the connection channel into a rear chamber of the piston chamber opposite to the pressure chamber. In this process, fluid will laterally past the piston of the overpressure valve.
Further, in the flow channel connecting the hydraulic pump to the pressure chamber, a control valve is provided. When the switch-off pressure has been reached, the resultant opening of the overpressure valve will cause a change of the pressure in the pressure chamber and thus also in the connection channel between hydraulic pump and pressure chamber. This will result in a switching of the control valve, resulting in the opening of a return flow channel. The return flow channel connects the pressure chamber to the fluid reservoir. The control valve as described in DE 20 2004 000 215 is of such a design that, during the pressing process, fluid will be pumped by the hydraulic pump through a narrow channel provided in the valve piston of the control valve. When the switch-off pressure has been reached and the pressure in the pressure chamber is thus sinking, the spring-biased valve piston of the control valve will be displaced and thus will clear the return flow channel. The process of pressing the pressure piston back into the starting position is performed by a return spring arranged in the pressure chamber. Said spring, when performing its return movement, will press the fluid in the pressure chamber—through the return flow channel cleared by the control valve—into the reservoir. A disadvantage of the pressing device described in DE 20 2004 000 215 resides in that, for performing the pressing process, the hydraulic fluid has to be pumped through the opening in the control valve that is small in cross section. This adversely affects the efficiency. Further, openings having a small cross section are susceptible to contamination.
An electro-hydraulic pressing device is also known from DE 198 25 160. In this device, a needle valve is arranged in the return flow channel. When the switch-off pressure has been reached, this needle valve will be opened so that the hydraulic fluid will be pressed back from the pressure chamber into the reservoir with the aid of the return spring. The needle valve arranged in the return flow channel comprises a channel having a small diameter, which during the pressing process is closed by the valve needle. Since also this device is provided with a channel of small cross section through which the fluid has to be pressed, the efficiency of the pressing device according to DE 198 25 160 in the process of pressing back the pressure piston is low. Further, there is again the disadvantage that the narrow-sectioned channel provided in the needle valve is easily contaminated.
It is the object of the disclosure to provide an electro-hydraulic pressing device which is suited particularly for producing tube connections by means of press fittings, wherein this device has a high efficiency both when opening and when closing the press jaws.